A novel millimeter-wave transition from microstrip to rectangular waveguide is introduced in this paper. The theory for this design is explained. The strip conductor on microstrip, which is introduced from the center of a rectangular waveguide broad-wall to its E-plane, is shaped into a triangular loop in the waveguide terminated by a short circuit. A back to back transition operating at Ka band was simulated and fabricated. The experimental results showed the insertion loss of a back to back transition is less than 2.2dB with its return loss greater than 9.1dB from 26.5 GHz to 35.5 GHz.